Recently, a file cloning technique in which a physical storage data of a replication source file stored in a file storage is shared with a file in a replication destination (clone file) is being utilized widely to enhance the speed of the file replication processing and the reduction of the storage device usage. Patent literature 1 discloses an art of creating a snapshot file that shares physical storage data with a replication source file and further creating a clone file for managing a difference data with respect to the snapshot file.
Further, patent literature 2 discloses an art of migration of a replication source file which shares physical storage data with the snapshot file to an offline storage.